Physical Review Applied

In the search for dark matter (DM), cavity-based experiments tune a microwave resonator to the unknown oscillation frequency of wavelike DM. Superconducting cavities would make these experiments far more sensitive, but wide-range tuning is needed. The authors mechanically separate the two halves of a Nb${}_3$Sn-coated cavity to realize a tuning range of more than 1 GHz around 8.5 GHz, while the quality factor Q remains above ${10}^6$. Simulations show that radiative losses stay negligible even for large openings, which makes the method a practical route toward wide-band experiments with high-Q cavities, even in significant magnetic fields.

Spin-polarized leptons are prized as probes of how spin shapes matter. Currently they are obtained from large accelerators or laser-driven sources that demand few-femtosecond synchronization and nanocoulomb-class electron beams. Using a laser-driven sheath field, the authors exploit the polarization effects in strong-field quantum electrodynamic processes to spin-polarize the injected electron beam and produce polarized positrons, with the two species separating automatically. This work enhances the experimental feasibility of lepton polarization, relaxes the requirements for the laser and electron beam, and opens a fresh route toward polarized lepton sources.

All-optical magnetization switching is attractive for ultrafast, energy-efficient magnetic information technologies, but most nanoscale demonstrations have relied on ferrimagnetic alloys, rather than ferromagnetic multilayers closer to spintronic device architectures. The authors investigate all-optical switching in patterned Pt/Gd/Co/Pt disks with just a little Gd in an otherwise ferromagnetic stack. Switching grows progressively less deterministic as disk diameter decreases, with an unexpected exponential size dependence arising from competition between weak switching bias and finite-size magnetization fluctuations—a fundamental constraint for future devices.

Superconducting transmon qubits are a cornerstone of solid-state quantum computing, yet their coherence is limited by dielectric loss in planar capacitors and junction electrodes. Here researchers explore to what extent this can be mitigated in mergemon qubits, which employ larger Josephson junctions that contribute most of the required capacitance. Using qubit spectroscopy to detect individual defects on surfaces and within tunnel junctions, the team compares different mergemon qubit designs and finds that careful balancing of junction and electrode geometries allows for long coherence times that are on par with similarly fabricated transmon qubits.

Topological phononic systems could enable backscattering-resistant control of mechanical waves for filters and interconnects in wireless and quantum technologies, but they are limited to low frequencies. The authors realize a gigahertz quantum spin Hall effect in nanoelectromechanical phononic crystals. They resolve the opposite handedness of the counterpropagating pseudospin branches in real space, and demonstrate a helical edge channel traversing two 120° corners without measurable loss. This brings topological phononics into the gigahertz regime, opening opportunities for robust phonon manipulation in integrated gigahertz circuits on a CMOS-compatible piezoelectric platform.

Remote magnetic tracking is crucial for navigation in GPS-denied environments, but real-world signals are often too weak, noisy, and complex to model reliably. The authors combine quantum diamond magnetometry with a deep-learning algorithm trained solely on experimental data to track an elevator in an urban setting. They show that full-vector magnetic inputs over short time windows yield robust centimeter-scale accuracy across arbitrary sensor frames despite strong noise, with model training requiring less than an hour. This model-free approach provides a practical route to deploying quantum magnetometry for real-time tracking in complex environments.

Understanding how interconnected neurons exchange information is a key scientific challenge, but existing optical methods are invasive, limiting recording durations. The authors combine microscopy with electrochromic optical recording (ECORE), a label-free technique relying on reversible color changes in a biocompatible medium. They demonstrate the recording of extracellular action potentials from cardiomyocytes with single-cell resolution and a high sensitivity of 6.3 µV. This approach simplifies the optical setup and allows simultaneous imaging, making ECORE accessible to a broader research community.

Fluxonium qubits are promising candidates for fault-tolerant quantum processors, but their dispersive readout remains challenging due to measurement-induced state transitions (MIST) that populate higher-excited states. To address this, the authors systematically explore MIST across a broad range of fluxonium parameters using branch analysis. They find that lighter fluxoniums are less susceptible to these transitions than heavier counterparts, and trace this to three key mechanisms. These results provide a path toward optimizing fluxonium design for fast, high-fidelity readout.

Nanodiamond sensors can measure magnetic fields at the nanoscale, but their random orientations create a calibration paradox: sensing the field requires knowing the orientation, while finding the orientation assumes the field is known. This is particularly prominent where one needs to separate the local field from the sample from the applied external bias field. This study shows that applying four non-coplanar bias fields breaks this deadlock, revealing both quantities at once. This self-calibration could enable nanodiamond sensing in complex environments such as living cells and aqueous solutions, where precalibrating the sensor is not an option.

Spatiotemporal acoustic vortices provide an intriguing route to encode transverse orbital angular momentum (OAM) in sound, but they have a fundamental constraint: Normally, their size grows with topological charge. This work overcomes that constraint by experimentally realizing perfect spatiotemporal acoustic vortices whose size remains nearly unchanged across different topological charges. The common vortex size also allows distinct OAM states to overlap and interfere on the same ring, producing programmable petal-like pressure patterns. These capabilities could facilitate acoustic multiplexing and detection, and structured-wave sensing and particle manipulation.

Spins in solids could provide long-lived memories for superconducting quantum computers, but practical devices need a microwave interface that remains tunable in a magnetic field. This study develops a superconducting resonator with adjustable frequency and bandwidth to address bismuth spins in silicon at a clock transition, where they are less sensitive to magnetic noise. The authors measure quantum coherence lasting nearly half a second and show that tuning the resonator can suppress the spins’ microwave echoes. These capabilities provide building blocks for controlling the storage and retrieval of quantum information in spin-based memories.

Accurate measurements of magnetic field direction are essential for applications ranging from magnetic navigation to space science, yet achieving high angular accuracy with compact optically pumped magnetometers (OPMs) remains challenging. The authors demonstrate a vector OPM based on radio-frequency-driven Zeeman Rabi oscillations in a microfabricated rubidium vapor cell. By combining calibrated radio-frequency fields with a Floquet-based atomic model, they achieve dead-zone-free vector measurements with a mean angular accuracy of 80 μrad using a single optical axis. This is a promising route toward compact, high-accuracy OPMs that can be integrated into existing architectures.

Chaotic magnetization dynamics in magnetic tunnel junctions (MTJs) could provide a compact physical platform for brain-inspired computing, but practical use requires electrical control and robustness against unavoidable thermal fluctuations. The authors use theory to show that spin-transfer torque generated by ac and dc currents enables both the excitation and control of chaos in a perpendicularly magnetized MTJ, with the dc current acting as an effective control parameter. Surprisingly, thermal fluctuations do not simply wash out the nonlinear dynamics; instead, they can promote noise-induced chaos.

Superconducting qubits are a leading platform for quantum computing, but scaling up their circuitry is limited by crosstalk between qubits, and by the growing density of control wiring. This theoretical study uncovers a particular kind of crosstalk: When the magnetic flux through one qubit varies in time, that qubit’s neighbors see this as a voltage source woven into the circuit itself, in a circuit version of Faraday’s law of induction. Rather than a bane, though, this crosstalk mechanism could be turned into a boon, enabling quantum gates with less wiring per qubit.

Periodic lattices of quantum emitters are promising platforms for quantum information processing, single-photon storage, and precision sensing. However, their performance is limited by an incomplete understanding of how intrinsic quantum nonlinearities shape their behavior beyond the linear (single-excitation) regime. The authors theoretically investigate coherently driven emitter lattices and show that quantum anharmonicity populates a continuum of excitonic Bloch states, including modes outside the light cone (subradiant), giving rise to broadband, incoherent emission that dominates under strong driving, and revealing an unexpected bistable regime controlled by effective driving renormalization.

The Dzyaloshinskii-Moriya (DM) interaction underlies chiral magnetic phenomena such as skyrmions, but its vectors are generally fixed by crystalline symmetry and difficult to manipulate. In this Letter the authors demonstrate control of DM-vector ordering in magnetic superlattices by mechanically breaking rotational and translational symmetries, using a strain technique. This fresh approach to engineering the DM interaction through control of structural symmetry seems to carry broad implications for chiral magnetism and spintronic applications.

Ultranarrow-linewidth lasers with suppressed high-frequency phase noise are critical for quantum control and precision metrology, but standard optical phase locking (OPL) is limited at high frequencies by feedback latency. This Letter presents a feedforward architecture that recycles the existing beat signal to drive a single electro-optic modulator, achieving robust phase-noise suppression that exceeds 30 dB from 10 kHz to 10 MHz. This approach is hardware-efficient, compatible with standard OPL setups, and readily extendable to gigahertz-level frequency offsets and optical frequency combs, offering a scalable route toward high-fidelity coherent control.

Although high-spin nuclear qudits in silicon offer a hardware-efficient route to quantum computation and error correction, scale-up is hindered by the weak, short-range interactions between nuclei. This Letter shows that electron spins can serve as a tunable quantum bus, mediating strong interactions between distant nuclear qudits and enabling high-fidelity two-qudit gates. The resulting coupling is robust against electron decoherence and charge noise while reaching a strength of about 100 kHz over distances exceeding 20 nm. This approach is a practical path toward scalable, electrically controlled qudit processors compatible with silicon-based quantum architectures.

Plasma photonic crystals (PPCs) are promising for controlling electromagnetic waves, but conventional systems with water electrodes suffer from strong microwave absorption. This study employs an electrode based on dimethyl silicone oil for dielectric barrier discharge, yielding a number of advantages that include significantly improved microwave signals. The system produces tunable triangular elements in a honeycomb superlattice that hosts robust valley-locked topological edge states. Here is a fresh approach to high-performance PPCs for integrated microwave devices and topological applications.

Extracting the current-phase relationship (CPR) of superconducting junctions in a noninvasive manner has proven to be difficult in the lab. The authors present an innovative method that overcomes the main limitations of conventional approaches, allowing the full harmonic content of the CPR to be extracted by subjecting the junction to a high-frequency ac drive. This method is demonstrated experimentally and found to be robust against experimental noise.

For electric field (E-field) sensing applications that require high sensitivity with very small sensing volume or high spatial resolution at low frequencies near dc, conventional electronic approaches suffer from low efficiency (due to small antenna size) and higher intrinsic noise. Using a simple vapor cell, innovative technical schemes, and an unexpected phenomenon reminiscent of magnetoresistance, the authors demonstrate a state-of-the-art quasi-dc atomic E-field sensor using “antenna-less” atoms. This approach is expected to enable electronic diagnostics, communication, proximity detection, charge-signature tracing, bioscience and geoscience research, and more.

$XY$ (planar-spin) Hamiltonians arise in phase synchronization and retrieval and analog formulations of hard optimization, which motivates fast, low-power physical solvers. However, gain-based $XY$ systems that encode each spin with a single complex field can become trapped in metastable states. The authors introduce an annealer that represents each spin with two coupled complex components, and uses a graph-independent locking term that exploits the extra degree of freedom to bypass barriers. For challenging graph families, this higher-dimensional annealing improves ground-state recovery compared to one-component approaches, supporting more reliable photonic and analog $XY$ optimization.

Materials that remain ventilating while providing airborne sound insulation are highly desirable for everyday noise controllers. This review discusses lattice metamaterials as an innovative class of advanced structures capable of fulfilling both functions. The authors survey sound-insulation performance and mechanisms, and identify the most effective types of architectures among distinct categories of lattice metamaterials. In addition, they propose numerical strategies to enable accelerated design exploration.